Driver second if not invested in hard work (5,3)

'driver' is the definition.
Although both the answer and definition are singular nouns, I don't understand how one could define the other.

'second if not invested in hard work' is the wordplay.
'second' becomes 'mo' (both are short periods of time).
'if not' becomes 'or' (both can mean 'otherwise...').
'invested in' means one lot of letters goes inside another.
'hard work' becomes 'toil' (I've seen this in another clue).
'or' going inside 'toil' is 'toroil'.
'mo'+'toroil'='MOTOR OIL'

Can you help me to learn more?